A CFD Modeling Coupled with VOF Method and Solidification Model for Molten Jet Breakup at Low Velocity

Abstract

In a reactor severe accident, molten jet breakup and solidification are important behaviors after large pours of molten material fall into the coolant in-vessel or ex-vessel. However, heat and mass transfer processes inside melt during jet breakup have not been studied sufficiently. Existing research on jet fragmentation is relatively macroscopic, and the micro interface condensation details are not well studied. In this paper, a two-dimensional multiphase computational fluid dynamics (CFD) code with the Volume of Fluid (VOF) method and solidification model is applied to simulate molten jet breakup with surface solidification. The VOF model is used to capture the interface, study the details, and add the influence of solidification. Solidification and instability can be seen at the interface. In order to simulate melt solidification, an energy equation is modeled using an enthalpy-based formulation, and viscosity variation during phase change is taken into account. The comparative results between the CFD code and jet breakup experiments show that melt jet front position histories, breakup length, and breakup time are in good agreement with the experiments. The simulation results show that crust formation of the jet surface suppresses surface instability and jet breakup behavior. As the interfacial temperature decreases, the droplet cumulative mass fraction decreases, and the solidified metal proportion increases. The simulation results by the CFD code with the solidification model are valuable and important for understanding the molten jet breakup mechanism.

Keywords:

• Molten jet
• numerical simulation
• Volume of Fluid method
• solidification
• breakup

Nomenclature

$C_P$ =	=	specific heat
$D,d$ =	=	diameter
$F_{\mathrm{l}}$ =	=	liquid fraction
$F_r$ =	=	Froude number
$g$ =	=	gravity
$h$ =	=	enthalpy
$K$ =	=	coefficient
$k$ =	=	curvature
$L$ =	=	length
$n$ =	=	unit normal vector
$p$ =	=	pressure
$S$ =	=	source term, coefficient
$T$ =	=	temperature
$t$ =	=	time
$u$ =	=	velocity in the x-axis direction
$v$ =	=	velocity in the y-axis direction
$V$ =	=	velocity

Greek

$\alpha$ =	=	fractional volume
$\mathrm{\Gamma }$ =	=	diffusion term
$\mathrm{\Delta }H$ =	=	latent heat for solidifying
$\delta$ =	=	Dirac distribution
$\lambda$ =	=	heat conductivity
$\mu$ =	=	dynamic viscosity
$\rho$ =	=	density
$\sigma$ =	=	surface tension
$\varphi$ =	=	variable

Subscript

$c$ =	=	coolant
$i$ =	=	phase index
$j$ =	=	jet
$l$ =	=	liquid
$m$ =	=	melt
$\mathrm{m}\mathrm{a}\mathrm{x}$ =	=	maximum
$s\hspace{0.17em}$ =	=	solid
$0$ =	=	initial

Disclosure Statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the Innovative Research Group Project of the National Natural Science Foundation of China [51506134, 11605193].